The present invention relates to an apparatus for applying tread material onto a tire.
A conventional apparatus for applying a tread onto a tire is described, for example, in EP-A1-637 506. In the process using this conventional apparatus for applying tread material onto a tire, below-the-surface openings in the outer surface of the carcass, which is the structure onto which the tread is to be applied, are filled to be level with the surrounding surface with overflow material comprising a binder rubber component, whereby the outer surface of the carcass is brought into a uniform surface level condition. Also, additional projections which are turned toward the flanks of the tire are built up with a binder rubber component onto the carcass to serve as well in rendering the outer surface of the carcass into a uniform surface level condition.
Very large tires including, in particular, tires for earth moving equipment, which together are hereinafter called EM tires and which have diameters between about two to three meters, can be subject to the chunking off of decidedly large pieces in their shoulder regions during the grinding off of the remaining tread especially if a roughly configured profile is present. This chunking off is attributable to the non-uniform force impact of the cutting knife through the tread profile projections as most such tires have decidedly hard tread profile blocks or lugs with correspondingly low elasticity which are generally first tilted before the cutting thereof is performed. During the grinding of just such tires, there occurs not only the chunking off of such pieces but also the occurrence of tears of large and small dimension and, indeed, this happens regularly in particular in the region of the shoulder of the carcass which is the location at which the tread profile is typically at its largest.
In view of the fact that such EM tires are high value and are correspondingly expensive products, the handling of such chunking off and tear damage in the shoulder regions of such tires has conventionally been by hand, whereby a repair person improves the respective deficiencies with a hand extruder. This work is decidedly time consuming in view of the large working surface. Even if the repair person applies effort, it is typical that a uniformly level outer surface cannot be obtained so that further labor must be expended; this further labor has conventionally typically comprised hand cutting with a rubber knife in order to thereby place the carcass in a relatively uniform round condition for the retreading operation.
It should be recognized that tears, which are not necessarily extremely small, involve some hand work to initially cut out around the tears and then fill the correspondingly enlarged area. This work is necessitated by the fact that binder rubber cannot effectively penetrate small and deep tears so that an attempt to introduce binder rubber into such tears would result in air being trapped within the tire carcass which would substantially reduce the quality of the retreaded tire.
The cutting out of the tears in the shoulder region of the carcass is, in any event, time consuming so that it is common to strike a compromise or trade-off and thus, on occasion, accept very small amounts of trapped air.
The challenge thus is, at the core, an urgent need to improve the decidedly laborious conventional manner to prepare a tire for a retreading operationxe2x80x94that is, to improve the steps between the grinding off of the tread profile, which is the so-called roughening of the carcass, and the application of a binder rubber layer on the roughened carcass.
It has also already been proposed to omit the hand labor steps to improve the outer surface of the carcass and instead to immediately iron out the binder rubber layer after the cutting out of the tears. However, the results of this approach have been less than satisfactory in that the areas below the surface in the tire shoulder are typically insufficiently filled and, additionally, small tears must be completely cut out to even have a realistic chance of maintaining a carcass relatively free of trapped air.
On the other hand, it would be desirable to avoid the necessity of having to cut out micro tears or relatively shallow tears.
The present invention offers an apparatus for applying tread material onto a tire as a solution to the challenge of providing an apparatus for applying tread material onto a tire which allows greater tolerance in the conditions of the carcass of the tire to be retread so that the preparation of the tire before the application of the new tread thereon is less critical and which additionally makes possible an improved quality of retreaded tires.
The apparatus for applying tread material onto a tire of the present invention provides the advantage that an intensive material application can be conducted precisely in the tear susceptible shoulder regions of the carcass. It is to be understood that, moreover, an especially good filling of cut out areas with an intensive connection of material to the underlying carcass material is possible without departing from the scope of the present invention. As a result of the focused and high pressure of the shoulder extruder of the apparatus for applying tread material onto a tire of the present invention, an intensive filling of the shoulder region is already ensured so that the relatively shallow tears do not require any cutting out work. It is also particularly beneficial that the apparatus for applying tread material onto a tire of the present invention is configured such that the applicator devices, which are at an angle to one another and accommodated to the flank and the tread surface of the carcass, optimally prepare the targeted outer surface in a known manner for the retreading operation and, in particular, that no out of roundness will occur. This is especially important in connection with EM tires since the detrimental influence of an out of round condition strongly increases in correspondence with the increase in tire diameter, whereby particular care must be taken with tires as large and expensive to produce as EM tires.
The tread surface which is obtainable by the use of the apparatus for applying tread material onto a tire of the present invention is significantly better than that obtainable by the conventional method of filling cut out areas by means of a hand extruder and cutting away the excess filling material with a hot knife.
In view of the fact that the tread surface which is obtainable by the use of the apparatus for applying tread material onto a tire of the present invention is correspondingly better, it is not necessary to perform the rougheningxe2x80x94that is, the grinding off of the existing tread profilexe2x80x94to such a deep extent. Accordingly, the risk is reduced that the belt in the carcass will become involved during the grinding off process. It is possible to perform the grinding off to a level only a few millimeters beyond that of the tread profile so that the thickness of the newly applied tread build up can be held to the minimum thickness possible. Since correspondingly less existing tread material needs to be ground off, the cutting speed of the cutting knife can be somewhat reduced, whereupon the tendency for tears and the deepening of openings is reduced or, alternatively, if the cutting speed of the cutting knife is maintained, the throughput of the retreading machine will increase.
In view of the fact that the angle included between the flank and the tread surface varies among the different EM tires, there are basically two alternate configurations of the apparatus for applying tread material onto a tire of the present invention which can accommodate the extruder of the apparatus for applying tread material onto a tire to the various included angles. In accordance with one alternate configuration, one of the two applicator devices can be configured so that the angle it forms with the other applicator device is adjustably variable, whereby the various included angles between the flank and the tread surface of the different EM tires can be accommodated. In accordance with the other alternate configuration, one of the two applicator devices is exchanged for another applicator device having a different fixed angular relationship to the non-exchanged applicator device. The fixed angle between the two applicator devices is typically about 110xc2x0.
In a particularly advantageous embodiment of the apparatus for applying tread material onto a tire of the present invention, it is provided that the extruder screw conveyor is controlled in response to pressure. In this connection, a pressure sensor is disposed adjacent the extruder nozzle and is in direct connection with the extruder screw conveyor in a control loop such that the most uniform possible pressure is maintained at the extruder nozzle. For example, a PID controller can be used for the desired control characteristic.
As a recess or depression which is to be filled comes under the extrusion nozzle during the rotation of the carcass, extrudate flows into the recess or depression, whereupon the pressure, which is being monitored by the pressure sensor, correspondingly decreases. In response, the extruder nozzle is accelerated. Once the filling of the recess or depression is at least partially completed, the pressure again increases whereupon the pressure sensor responsively effects a reduction in the rotational speed of the extruder screw conveyor. During the passage past the extruder nozzle of areas of the shoulder of the tire which are free of areas requiring filling, the extruder screw conveyor is controlled by the pressure sensor to remain in a non-feeding condition or is controlled to rotate at only a very small speed so that a smeared layer of extrudate is applied onto the carcass as a coherent film which, for example, can be less than 1 mm thick and which serves as well in the preparation of the tire carcass for the subsequent retreading of the tire.
If the extrudate comprises binder rubber, it is possible to achieve a particularly good adhesion of the tread bands applied onto the shoulder regions of the tire without requiring the special attention which must normally be paid during the application of the binder rubber layer in a CTT- or CTC process handling of the shoulders due to the customary large problems associated therewith.
In view of the fact that a very thin binder rubber layer is sufficient, it is possible to realize a corresponding decidedly economic preparation of the tire for its subsequent retreading.
In correspondence with the type of recess or depression on the carcass, a shoulder cavity can exhibit a depth up to 10 cm and a volume of 1 liter. It is to be understood, however, that it is possible, in accordance with the automatic filling of the cavities in accordance with the present invention, to effect an accommodation of a wide range of cavity sizes.